Synthesis of ceramic superconductor CPO from xenotyme

ABSTRACT

A process for the preparation of a superconductor of the type AB 2  Cu 3  O 7-x , wherein A is a mixture of rare earths, B is an alkaline earth metal and x is less than one, comprising the treatment of xenotime to obtain a rare earths oxide therefrom and mixing the oxide with an alkaline earth metal compound and a copper compound and heat treating the mixture to obtain the superconductor. The treatment of the xenotime comprising melting the xenotime with a 1:1 mixture of Na 3  CO 3  and K 2  CO 3 , dissolving a solid obtained therefrom in HCl, treating the resulting solution with NH 4  OH to precipitate the rare earths, dissolving the precipitate in HCl, treating the resulting solution with H 2  C 2  O 4  to precipitate the rare earths aned calcinating the precipitate to obtain a rare earths oxide.

BACKGROUND OF THE INVENTION

Superconductive materials are those where the electric resistance fallsto zero (10⁻²⁰ Ohm/cm) below a critical temperature, its value dependingon the material. The superconductivity is defined only within a criticalsurface (only figure), its axes being the temperature, the electricalcurrent and the magnetic field. Thus, for a given working temperaturethere is a defined curve of critical current in function of the magneticfield generated and/or applied to the superconductor.

The main practical superconductor materials are NbTi and Nb₃ SN. Theirworking temperature is only 4.2K, which is the boiling temperature ofliquid helium. This is the main limitation to large scale application ofsuperconductivity. Superconductors are being used almost exclusively forwinding of magnets. Manufactured from wires (NbTi and Nb₃ Sn) or tapes(Nb₃ Sn) with high critical current densities (3500 A/mm² at 5 Tesla forNbTi), they allow the winding of compact magnets for the production ofhigh fields (up to 18 Tesla) in large volumes.

The most popular superconductor magnets are the ones used for theformation of medical images by nuclear magnetic resonance (MRI) and formaterials analysis by the same principle (NMR), the magnets for oreseparation and research magnets for high fields. Among the latter arethe magnets used in large particle accelerators (SSC, HERA, KEK, etc.).

The superconductor magnets are included in the conceptual project oflarge machines, among them nuclear fusion reactors, energy generatorsfor magneto-hydro-dynamics (MHD) and magnets for energy storage in spacestations. Of course, some difficulties in the feasibility of these largemachines transcend superconductivity.

The oxide superconductors of high critical temperature were discoveredin 1986. These are intermetallic compounds involving copper oxide andrare earths, with perovskite (mica) crystal structure. Their criticaltemperatures vary from 30K to room temperature and their critical fieldsare above 60 Tesla. Therefore these materials are considered verypromising and may replace with advantages the Nb₃ Sn and NbTi in themanufacture of magnets and find other applications not feasible withliquid helium, such as transmission of electricity, for example. Thesematerials are not yet on the market as wires, cables, films, tapes orsheets.

SUMMARY OF THE INVENTION

The present invention relates to the synthesis of superconductors fromxenotime, which is an yttrium phosphate mineral common in Brazilcontaining only rare earth producing good oxide superconductors.

The rare earth oxides represent 45 to 55% of the weight of the mineral.The preferred, but not exclusive destination of these superconductors isthe winding of magnets, transmission and distribution of electricity,magnetic shielding, computer applications (hybrid superconductivesemiconductors, Josephson joints, transistors, etc.), among otherapplications.

BRIEF DESCRIPTIONS OF DRAWINGS

The attached FIGURE illustrates the critical surface relative tosuperconductivity, indicated by (A).

DETAILED DESCRIPTIONS OF THE INVENTION

The following is an example of the above exposure: 100 g of pulverizedxenotime (200 mesh) are melted at 1000° C. with 1000 g of a mixture ofNa₂ CO₃ /K₂ CO₃ in proportion of 1:1. The resulting solid is rinsed with2 liters of a solution of 1:10 of Na₂ CO₃, then treated with 200 ml ofHCl 1M at 60° C. and then filtered. The residue is discarded. Thesolution is treated with NH₄ OH 3M until all rare earths areprecipitated, an excess of NH₄ OH is added to separate certain cationswhich form soluble complexes with NH₄ OH (Zn² +, Cu² +, Ni² +, etc.)which may be present. The precipitate is dissolved with HCl 3M and againprecipitated with oxalic acid at a temperature between 50° and 60° C.The precipitate is rinsed with diluted oxalic acid and calcinated at850° C. The mixture of oxides, of a mass between 45 and 55 g containsonly the rare earths originally present in the 100 g of xenotime.

In order to obtain 100 g of superconductor, a mixture is prepared from15 g of rare earth oxides obtained as per above, 53 g of bariumcarbonate or other alkaline earth and 32 g of copper oxide. A heattreatment is performed for the reaction of the above components at 950°C. for several hours. Then the calcinated material is ground and againheat treated for some hours at below 1000° C. to allow for control bychemical calculation. Then the material is ground again to allow forgranulometric control.

What is claimed is:
 1. A process for the preparation of a superconductorof the type AB₂ Cu₃ O_(7-x), where x is less than one, A is a mixture ofrare earths and B is an alkaline earth metal, consisting essentially ofthe steps of:(a) melting xenotime with a 1:1 mixture of Na₂ CO₃ and K₂CO₃ cooling the melt to obtain a solid; (b) dissolving the solidobtained from step (a) in HCl; (c) treating the resulting solution ofstep (b) with NH₄ OH to precipitate the rare earth to form a solution;(d) dissolving the resulting rare earth precipitate of step (c) in HClto form a solution; (e) treating the resulting solution of step (d) withoxalicacid (H₂ C₂ O₄) to precipitate the rare earth; (f) calcinating theresulting rare earth precipitate of step (e) to obtain rare earth oxide;(g) mixing said rare earth oxide with an alkaline earth metal compoundand a copper oxide; and (h) heat treating the resultant mixture of step(g) to obtain said superconductor.
 2. A process for the preparation of asuperconductor of the type AB₂ Cu₃ O_(7-x), where x is less than one, Ais a mixture of rare earths and B is an alkaline earth metal, consistingessentially of the steps of:(a) melting the xenotime at 1000° C. with aflux of a 1:1 mixture of Na₂ CO₃ and K₂ CO₃ cooling the melt to obtain asolid; (b) washing the resulting solid of step (a) in a solution of Na₂CO₃ ; (c) treating the washed solid of step (b) with HCl to dissolve thesolid to form a solution; (d) filtering the resulting solution of step(c); (e) treating filtered solution of step (d) with NH₄ OH toprecipitate the rare earth; (f) dissolving the resulting precipitate ofstep (e) in HCl to form a solution; (g) treating the resulting solutionof step (f) with oxidic acid (H₂ C₂ O₄) at about 50° to 60° C. resultingin a rare earth precipitate; (h) washing the resulting precipitate ofstep (g) with H₂ C₂ O₄ ; (i) calcinating said precipitate at about 850°C. to obtain a rare earth oxide; (j) mixing said rare earth oxide withan alkaline earth metal compound and a copper oxide; and (k) heattreating the resultant mixture of step (j) to obtain saidsuperconductor.